Document Type

Data

Publication Date

7-1-2026

Abstract

Clouds act as crucial regulators of the Earth's radiation budget by reflecting shortwave solar radiation and absorbing longwave thermal emission. However, they continue to represent a major source of uncertainty in the climate models due to the subgrid-scale processes of cloud microphysics. A particularly complex mechanism is dry-air entrainment at cloud boundaries, which alters local supersaturation levels and drives droplet activation-deactivation cycles. Despite its significance, capturing these fast, small-scale processes remains an ongoing challenge.

In-situ airborne instruments provide only localized, intermittent spatial sampling along flight paths, whereas traditional ground-based remote sensing techniques lack the fine spatial resolution necessary to observe microphysical variability at centimeter scales. To address these observational limitations, this study utilizes an advanced laboratory setup in the Pi cloud convection chamber to explore the coupled effects of dry-air entrainment and aerosol loading on the vertical profile of clouds. Microphysical observations are achieved by pairing a high-resolution Time-Correlated Single Photon Counting (TCSPC) lidar, capable of sub-meter profiling with a 1.5 cm range resolution, with a WELAS optical particle counter. A novel retrieval algorithm is developed to derive vertical extinction (σ) profiles. Four distinct steady-state cloudy conditions, clean-background, clean-entrainment, polluted-background, and polluted-entrainment, are generated via moist Rayleigh-Bénard convection under weak thermodynamic forcing. Dry, aerosol-free air is introduced from the top of the chamber to simulate entrainment and observe subsequent structural changes.

The results show that while dry-air entrainment drives droplet evaporation and reduces liquid water content across all regimes, the change of vertical structure is highly dependent on aerosol concentration. In clean clouds, the system exhibits a buffering capacity; water vapor fluxes from the chamber floor effectively replenish supersaturation, confining entrainment-driven changes to the upper layer. Conversely, polluted clouds transition into a water-vapor-limited, fluctuation-dominated regime. Intense competition for limited vapor suppresses supersaturation near zero, accelerating droplet deactivation. Driven by turbulent mixing and localized reactivation-deactivation cycles, the haze number concentration significantly increases, particularly near the bottom boundary. This interaction induces vertical inhomogeneity and a distinct downward increase in extinction. These high-resolution findings provide important process-level evidence of aerosol-modulated entrainment sensitivity, offering a scalable framework applicable to boundary layer fog and microphysical cloud parameterizations.

Comments

Funding Sources

US National Science Foundation under grants AGS-2133229 and AGS-2113060.

S. P. Singh was supported by Simons Foundation grant PD-Grant-01249402.

F. Yang was supported by the DOE-SC BER program under contract DE-SC0012704.

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